Theoretical bonding, poly

In spite of the considerable difference in experimental and theoretical results obtained by different authors for the dimensions of macromolecules of aromatic polyamides, these results indicate a sharp asymmetry of the macromolecules. It should be pointed out, for the sake of comparison, that for flexible chain polymers the Kuhn segment amounts to only 20-50 A. For example, for an aromatic polyamide which does not exhibit coaxial rotation of the bonds, poly(m-phenylene isophtalamide), the Kuhn segment is equal to 50 A, i.e. it is 10-15 times as small as that for its para-analog. 

The 1,1-disubstitution of chlorine atoms causes steric interactions in the polymer, as is evident from the heat of polymeri2ation (see Table 1) (24). When corrected for the heat of fusion, it is significantly less than the theoretical value of —83.7 kJ/mol (—20 kcal/mol) for the process of converting a double bond to two single bonds. The steric strain apparentiy is not important in the addition step, because VDC polymeri2es easily. Nor is it sufficient to favor depolymeri2ation the estimated ceiling temperature for poly (vinyhdene chloride) (PVDC) is about 400°C. 

The Griffith crack equation has been shown to apply, albeit with some scatter of results, to the brittle polymeric materials poly(methyl methacrylate) and poly(styrene) when cracks of controlled size have been introduced deliberately into the specimens. Such experiments give values of surface energy that are very large, typically 10 - 10 J m , which is about 100 times greater than the theoretical value calculated from the energy of the chemical bonds involved. This value of y thus seems to be made up of two terms, Le. 

As mentioned previously, the main drawbacks of the thermal route to poly-borylborazine are (1) the presence of both direct intercyclic bonds and three-atom bridges between the rings, and (2) a difficulty in controlling the polycondensation rate. One solution we investigated to address these drawbacks is a route based on the room temperature reaction of /i-chloroborazine with trialkylaminoborane.31 32 We used 2-methylamino-4,6-dichloroborazine instead of 2,4,6-trichloroborazine to prepare a two-point polymer (scheme 4), which is theoretically less cross-linked. 

A series of compounds that has been studied extensively, both theoretically and experimentally, is the poly phenyls (38-40). In considering these compounds it is very helpful if one has at least a little understanding of the significance of the X-ray crystallographic results. Biphenyl, the first of the series, is known to be nonplanar in the gas phase, with a torsional angle about the central bond of 42°. This is interpreted in terms of relief of steric interference between the... 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

Helix. The a> helix possesses a fourfold axis of symmetry, while retaining the i to i+4 hydrogen bonding scheme [77, 78], Therefore, it is looser helical structure than the a helix, and has been identified in forms of poly (L-Phe) [79] and poly (benzyl-L-Asp) [80]. The canonical 6 and y angles are -38° and -78°, respectively. No experimental CD spectra have been reported, but a theoretical CD has been computed [38]. 

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